This behavior could be attributed to the dog's urine having a sweet taste associated with diabetes or it could be a learned behavior from his mother. A nursing mother will ingest the feces and urine of her puppies to keep the nesting area clean. Some dogs do "learn" the behavior from thier mother. You can discourage the behavior by keeping the urine cleaned up and if you notice him start to do this, give him a correction in the form of a low toned "NO". Hopefully he will outgrow this. I would mention it to your vet when you take him in for his vaccination so he can be checked for diabetes.